Method for enhanced evanescent field exposure in an optical fiber resonator for spectroscopic detection and measurement of trace species

ABSTRACT

A method for detection and measurement of trace species in a gas or liquid sample is provided. The method comprises forming a sensor from an optical fiber by tapering a portion the optical fiber along a length thereof, exposing the tapered portion of the optic fiber to the sample gas or sample liquid, emitting radiation from a coherent source, coupling at least a portion of the radiation emitted from the coherent source into the fiber optic ring, receiving a portion of the radiation traveling in the fiber optic ring, and determining the level of trace species in the gas or liquid sample based on a rate of decay of the radiation within the fiber optic ring.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/157,400, filed May 29, 2002 now U.S. Pat. No. 7,318,909,which is a Continuation-in-Part of U.S. patent application Ser. No.10/017,367 filed on Dec. 12, 2001 now U.S. Pat. No. 7,046,362.

FIELD OF THE INVENTION

This invention relates generally to absorption spectroscopy and, inparticular, is directed to fiber optic sensors having enhancedevanescent field regions for use with a fiber-optic resonator forring-down cavity spectroscopy.

BACKGROUND OF THE INVENTION

Referring now to the drawing, wherein like reference numerals refer tolike elements throughout, FIG. 1 illustrates the electromagneticspectrum on a logarithmic scale. The science of spectroscopy studiesspectra. In contrast with sciences concerned with other parts of thespectrum, optics particularly involves visible and near-visible light—avery narrow part of the available spectrum which extends in wavelengthfrom about 1 mm to about 1 nm. Near visible light includes colors redderthan red (infrared) and colors more violet than violet (ultraviolet).The range extends just far enough to either side of visibility that thelight can still be handled by most lenses and mirrors made of the usualmaterials. The wavelength dependence of optical properties of materialsmust often be considered.

Absorption-type spectroscopy offers high sensitivity, response times onthe order of microseconds, immunity from poisoning, and limitedinterference from molecular species other than the species under study.Various molecular species can be detected or identified by absorptionspectroscopy. Thus, absorption spectroscopy provides a general method ofdetecting important trace species. In the gas phase, the sensitivity andselectivity of this method is optimized because the species have theirabsorption strength concentrated in a set of sharp spectral lines. Thenarrow lines in the spectrum can be used to discriminate against mostinterfering species.

In many industrial processes, the concentration of trace species inflowing gas streams and liquids must be measured and analyzed with ahigh degree of speed and accuracy. Such measurement and analysis isrequired because the concentration of contaminants is often critical tothe quality of the end product. Gases such as N₂, O₂, H₂, Ar, and He areused to manufacture integrated circuits, for example, and the presencein those gases of impurities—even at parts per billion (ppb) levels—isdamaging and reduces the yield of operational circuits. Therefore, therelatively high sensitivity with which water can be spectroscopicallymonitored is important to manufacturers of high-purity gases used in thesemiconductor industry. Various impurities must be detected in otherindustrial applications. Further, the presence of impurities, eitherinherent or deliberately place, in liquids have become of particularconcern of late.

Spectroscopy has obtained parts per million (ppm) level detection forgaseous contaminants in high-purity gases. Detection sensitivities atthe ppb level are attainable in some cases. Accordingly, severalspectroscopic methods have been applied to such applications asquantitative contamination monitoring in gases, including: absorptionmeasurements in traditional long pathlength cells, photoacousticspectroscopy, frequency modulation spectroscopy, and intracavity laserabsorption spectroscopy. These methods have several features, discussedin U.S. Pat. No. 5,528,040 issued to Lehmann, which make them difficultto use and impractical for industrial applications. They have beenlargely confined, therefore, to laboratory investigations.

In contrast, cavity ring-down spectroscopy (CRDS) has become animportant spectroscopic technique with applications to science,industrial process control, and atmospheric trace gas detection. CRDShas been demonstrated as a technique for the measurement of opticalabsorption that excels in the low-absorbance regime where conventionalmethods have inadequate sensitivity. CRDS utilizes the mean lifetime ofphotons in a high-finesse optical resonator as the absorption-sensitiveobservable.

Typically, the resonator is formed from a pair of nominally equivalent,narrow band, ultra-high reflectivity dielectric mirrors, configuredappropriately to form a stable optical resonator. A laser pulse isinjected into the resonator through a mirror to experience a meanlifetime which depends upon the photon round-trip transit time, thelength of the resonator, the absorption cross section and number densityof the species, and a factor accounting for intrinsic resonator losses(which arise largely from the frequency-dependent mirror reflectivitieswhen diffraction losses are negligible). The determination of opticalabsorption is transformed, therefore, from the conventional power-ratiomeasurement to a measurement of decay time. The ultimate sensitivity ofCRDS is determined by the magnitude of the intrinsic resonator losses,which can be minimized with techniques such as superpolishing thatpermit the fabrication of ultra-low-loss optics.

At present, CRDS is limited to spectroscopic regions where highreflectivity dielectric mirrors can be used. This has significantlylimited the usefulness of the method in much of the ultraviolet andinfrared regions, because mirrors with sufficiently high reflectivityare not presently available. Even in regions where suitable dielectricmirrors are available, each set of mirrors only allows for operationover a small range of wavelengths, typically a fractional range of a fewpercent. Further, construction of many dielectric mirrors requires useof materials that may degrade over time, especially when exposed tochemically corrosive environments. Because these present limitationsrestrict or prevent the use of CRDS in many potential applications,there is a clearly recognized need to improve upon the current state ofthe art with respect to resonator construction.

The article by A. Pipino et al., “Evanescent wave cavity ring-downspectroscopy with a total-internal reflection minicavity,” Rev. Sci.Instrum. 68 (8) (August 1997), presents one approach to an improvedresonator construction. The approach uses a monolithic, total internalreflection (TIR) ring resonator of regular polygonal geometry (e.g.,square and octagonal) with at least one convex facet to inducestability. A light pulse is totally reflected by a first prism locatedoutside and in the vicinity of the resonator, creating an evanescentwave which enters the resonator and excites the stable modes of theresonator through photon tunneling. When light impinges on a surface oflower index of refraction that the propagation medium at greater than acritical angle, it reflects completely. J. D. Jackson, “ClassicalElectrodynamics,” Chapter 7, John Wiley & Sons, Inc.: New York, N.Y.(1962). A field exists, however, beyond the point of reflection that isnon-propagating and decays exponentially with distance form theinterface. This evanescent field carries no power in a pure dielectricmedium, but attenuation of the reflected wave allows observation of thepresence of an absorbing species in the region of the evanescent field.F. M. Mirabella (ed.), “Internal Reflection Spectroscopy,” Chapter 2,Marcel Dekker, Inc.: New York, N.Y. (1993).

The absorption spectrum of matter located at the totally reflectingsurfaces of the resonator is obtained from the mean lifetime of a photonin the monolithic resonator, which is extracted from the time dependenceof the signal received at a detector by out coupling with a second prism(also a totally reflecting prism located outside, but in the vicinityof, the resonator). Thus, optical radiation enters and exits theresonator by photon tunneling, which permits precise control of inputand output coupling. A miniature-resonator realization of CRDS resultsand the TIR-ring resonator extends the CRDS concept to condensed matterspectroscopy. The broadband nature of TIR circumvents the narrowbandwidth restriction imposed by dielectric mirrors in conventionalgas-phase CRDS. The work of A. Pipino et al. is only applicable to TIRspectroscopy, which is intrinsically limited to short overall absorptionpathlengths, and thus powerful absorption strengths. In contrast, thepresent invention provides long absorption pathlengths and thus allowsfor detection of weak absorption strengths.

Various novel approaches to mirror based CRDS systems are provided inU.S. Pat. Nos. 5,973,864, 6,097,555, 6,172,823 B1, and 6,172,824 B1issued to Lehmann et al., and incorporated herein by reference. Theseapproaches teach the use of a near-confocal resonator formed by tworeflecting elements or prismatic elements.

FIG. 2 illustrates a prior art CRDS apparatus 10. As shown in FIG. 2,light is generated from a narrow band, tunable, continuous wave diodelaser 20. Laser 20 is temperature tuned by a temperature controller 30to put its wavelength on the desired spectral line of the analyte. Anisolator 40 is positioned in front of and in line with the radiationemitted from laser 20. Isolator 40 provides a one-way transmission path,allowing radiation to travel away from laser 20 but preventing radiationfrom traveling in the opposite direction. Single mode fiber coupler(F.C.) 50 couples the light emitted from laser 20 into the optical fiber48. Fiber coupler 50 is positioned in front of and in line with isolator40. Fiber coupler 50 receives and holds optical fiber 48 and directs theradiation emitted from laser 20 toward and through a first lens 46.First lens 46 collects and focuses the radiation. Because the beampattern emitted by laser 20 does not perfectly match the pattern oflight propagating in optical fiber 48, there is an inevitable mismatchloss.

The laser radiation is approximately mode-matched into a ring downcavity (RDC) cell 60. A reflective mirror 52 directs the radiationtoward a beam splitter 54. Beam splitter 54 directs about 90%, of theradiation through a second lens 56. Second lens 56 collects and focusesthe radiation into cell 60. The remaining radiation passes through beamsplitter 54 and is directed by a reflective mirror 58 into an analytereference cell 90.

The radiation which is transmitted through analyte reference cell 90 isdirected toward and through a fourth lens 92. Fourth lens 92 is alignedbetween analyte reference cell 90 and a second photodetector 94 (PD 2).Photodetector 94 provides input to computer and control electronics 100.

Cell 60 is made from two, highly reflective mirrors 62, 64, which arealigned as a near confocal etalon along an axis, a. Mirrors 62, 64constitute the input and output windows of cell 60. The sample gas understudy flows through a narrow tube 66 that is coaxial with the opticalaxis, a, of cell 60. Mirrors 62, 64 are placed on adjustable flanges ormounts that are sealed with vacuum tight bellows to allow adjustment ofthe optical alignment of cell 60.

Mirrors 62, 64 have a high-reflectivity dielectric coating and areoriented with the coating facing inside the cavity formed by cell 60. Asmall fraction of laser light enters cell 60 through front mirror 62 and“rings” back and forth inside the cavity of cell 60. Light transmittedthrough rear mirror 64 (the reflector) of cell 60 is directed toward andthrough a third lens 68 and, in turn, imaged onto a first photodetector70 (PD 1). Each of photodetectors 70, 94 converts an incoming opticalbeam into an electrical current and, therefore, provides an input signalto computer and control electronics 100. The input signal represents thedecay rate of the cavity ring down.

FIG. 3 illustrates optical path within a prior art CRDS resonator 100.As shown in FIG. 3, resonator 100 for CRDS is based upon using twoBrewster's angle retroreflector prisms 50, 52. The polarizing orBrewster's angle, Θ_(B), is shown relative to prism 50. Incident light12 and exiting light 14 are illustrated as input to and output fromprism 52, respectively. The resonant optical beam undergoes two totalinternal reflections without loss in each prism 50, 52 at about 45°, anangle which is greater than the critical angle for fused quartz and mostother common optical prism materials. Light travels between prisms 50,52 along optical axis 54.

Although, when compared with the other spectroscopy methods, ring downcavity spectroscopy is a simpler and less expensive to implement, it isstill costly in that a ring down cavity spectroscopy system can cost onthe order of many thousands of dollars per unit. In addition,conventional CRDS devices are prone to misalignment between the opticalelements while being fabricated as well as during use.

To overcome the shortcomings of the known approaches to improvedresonator construction, a new optic-fiber based optical resonator forCRDS is provided. An object of the present invention is to replaceconventional fiber optic sensors with sensors having enhanced evanescentfield portion, thereby providing a more sensitive fiber optic sensor.

SUMMARY OF THE INVENTION

To achieve that and other objects, and in view of its purposes, thepresent invention provides an improved apparatus for trace speciesdetection and measurement in a sample gas. The apparatus includes apassive fiber optic cable; at least one sensor in line with the fiberoptic cable, the at least one sensor having a portion thereof exposed tothe sample gas or sample liquid; a coherent source of radiation;coupling means for i) introducing a portion of the radiation emitted bythe coherent source to the passive fiber optic ring and ii) receiving aportion of the resonant radiation in the passive fiber optic ring; adetector for detecting a level of the radiation received by the couplingmeans and generating a signal responsive thereto; and a processorcoupled to the detector for determining a level of the trace species inthe gas sample or liquid sample based on the signal generated by thedetector.

According to another aspect of the invention, the sensor has a taperedportion exposed to the sample gas or sample liquid.

According to a further aspect of the invention, the sensor has anexposed portion with a “D” shaped cross section.

According to yet another aspect of the invention, the level of the tracespecies is determined based on a rate of decay of the signal generatedby the detector means.

According to a further aspect of the invention, a filter is placedbetween the coupling means and the detector to selectively pass thereceived portion of radiation from the passive fiber optic loop to thedetector.

According to yet another aspect of the invention, the coupler includesi) a first coupler for introducing the portion of the radiation emittedby the coherent source to a first section of the fiber optic ring andii) a second coupler for receiving the portion of the radiation in thepassive fiber optic ring at a second section thereof.

According to still another aspect of the invention, the exposed portionof the fiber is the cladding of the fiber.

According to yet a further aspect of the invention, the exposed portionof the fiber is the inner core of the fiber.

According to another aspect of the invention, the coherent source is anoptical parametric generator, an optical parametric amplifier, or alaser.

According to yet another aspect of the invention, an evanescent field ofthe radiation traveling within the fiber is exposed to the sample gas orsample liquid.

According to still another aspect of the invention, the absorption ofthe radiation from the fiber increases a rate of decay of the radiation.

According to yet a further aspect of the invention, the passive resonantfiber has a hollow core.

According to yet another aspect of the invention, the apparatus furthercomprises a sensor formed from a cylindrical body and wrapped with asection of the exposed portion of the resonant fiber such that exposureof the evanescent field to the trace species is enhanced by increasingthe penetration depth of the evanescent field.

According to a further aspect of the invention, at least a portion ofthe passive fiber optic ring is coated with a material to selectivelyincrease a concentration of the trace species at the coated portion ofthe fiber optic ring.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 illustrates the electromagnetic spectrum on a logarithmic scale;

FIG. 2 illustrates a prior art CRDS system using mirrors;

FIG. 3 illustrates a prior art CRDS cell using prisms;

FIG. 4 is an illustration of a first exemplary embodiment of the presentinvention;

FIG. 5A is a end view of a conventional optical fiber;

FIG. 5B is a perspective view of a sensor according to an exemplaryembodiment of the present invention;

FIG. 6A is a cross sectional view of fiber optic cable illustratingpropagation of radiation within the cable;

FIG. 6B is a cross section of a fiber optic sensor illustrating theevanescent field according to an exemplary embodiment of the presentinvention

FIG. 6C is a cross section of a fiber optic sensor illustrating theevanescent field according to another exemplary embodiment of thepresent invention;

FIG. 6D is a cross-section of a fiber optic sensor according to anotherexemplary embodiment of the present invention;

FIG. 7 is an illustration of a second exemplary embodiment of thepresent invention;

FIGS. 8A-8D are illustrations of a fiber optic sensor according to athird exemplary embodiment of the present invention;

FIGS. 9A-9C are illustrations of a fiber optic sensor according to afourth exemplary embodiment of the present invention; and

FIGS. 10A-10C are illustrations of a fiber optic sensor according to afifth exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The entire disclosure of U.S. patent application Ser. No. 10/017,367filed Dec. 12, 2001 is expressly incorporated herein by reference.

FIG. 4 illustrates fiber optic based ring-down apparatus 400 accordingto a first exemplary embodiment of the present invention through whichtrace species, or analytes, in gases and liquids may be detected. InFIG. 4, apparatus 400 includes resonant fiber optic ring 408 which hasfiber optic cable 402 and sensors 500 (described below in detail)distributed along the length of fiber optic cable 402. The length ofresonant fiber optic ring 408 is easily adaptable to a variety ofacquisition situations, such as perimeter sensing or passing throughvarious sections of a physical plant, for example. Although as shown,sensors 500 are distributed along the length of fiber optic loop 408,the invention may be practiced using only one sensor 500, if desired.The distribution of more than one sensor 500 allows for sampling of atrace species at various points throughout the installation site. Theinvention may also be practiced using a combination of sensors 500 withstraight section of fiber 402 exposed to sample liquids or gases, orwith only straight sections of fiber 402 exposed to the sample liquid orgas. It is contemplated that the length of resonant fiber optic ring maybe as small as about 1 meter or as large as several kilometers.

Coherent source of radiation 404, such as an optical parametricgenerator (OPG), optical parametric amplifier (OPA) or a laser, forexample, emits radiation at a wavelength consistent with an absorptionfrequency of the analyte or trace species of interest. Coherent source404 may be a tunable diode laser having a narrow band based on the tracespecies of interest. An example of a commercially available opticalparametric amplifier is model no. OPA-800C available from SpectraPhysics, of Mountain View, Calif.

Examples of frequencies of coherent source 404 versus analytes areoutlined in Table 1. Table 1 is merely illustrative and not intended asrestrictive of the scope of the present invention. Further, it iscontemplated that the present invention may be used to detect a varietyof chemical and biological agents harmful to humans and/or animals. Itis also contemplated that such detection may be enhanced by coating thesurface of the passive fiber optic ring with antibodies thatspecifically bind the desired antigen.

TABLE 1 Approximate Approximate Analyte or Wavelength(s) Wavelength(s)Trace Species Near Infrared Mid Infrared Water (H2O) 1390 nm 5940 nmAmmonia (NH3) 1500 nm 10300 nm  Methane (CH4) 1650 nm 3260 nm CarbonDioxide (CO2) 1960 nm 4230 nm Carbon Monoxide (CO) 1570 nm; 2330 nm 4600nm Nitric Oxide (NO) 1800 nm; 2650 nm 5250 nm Nitrogen Dioxide (NO2)2680 nm 6140 nm Nitrous Oxide (N2O) 2260 nm 4470 nm Sulfur Dioxide (SO2)7280 nm Acetylene 1520 nm 7400 nm Hydrogen Fluoride (HF) 1310 nmHydrogen Chloride (HCl) 1790 nm 3400 nm Hydrogen Bromide (HBr) 1960 nm3820 nm Hydrogen Iodide (HI) 1540 nm Hydrogen Cyanide (HCN) 1540 nm 6910nm Hydrogen Sulfide (H2S) 1570 nm Ozone (O3) 9500 nm Formaldehyde (H2CO)1930 nm 3550 nm Phosphine (PH3) 2150 nm 10100 nm  Oxygen (O2)  760 nm

In the first exemplary embodiment, radiation from coherent source 404 isprovided to resonant fiber optic ring 408 through optional opticalisolator 406, coupler 410, and evanescent input coupler 412. Whencoherent source 404 is a diode laser, using optical isolator 406provides the benefit of minimizing noise in the laser by preventingreflections back into the laser. Evanescent input coupler 412 mayprovide a fixed percentage of radiation from coherent source 404 intoresonant fiber optic ring 408, or may be adjustable based on lossespresent throughout resonant fiber optic ring 408. Preferably, the amountof radiation provided by evanescent input coupler 412 to resonant fiberoptic ring 408 matches the losses present in fiber optic cable 402 andthe connectors (not shown). A commercially available evanescent couplerproviding 1% coupling (99%/1% split ratio coupling) of radiation ismanufactured by ThorLabs of Newton, N.J., having part number 10202A-99.In a preferred embodiment, evanescent input coupler 412 couples lessthat 1% of the radiation from coherent source 404 into fiber 402.

In one exemplary embodiment, to detect the trace species or analyte, aportion of the jacket 402 a covering the fiber optic cable 402 isremoved to expose cladding 402 b that surrounds inner core 402 c offiber optic cable 402. Alternatively, either both jacket 402 a andcladding 402 b may be removed to expose inner core 402 c, or thejacketed portion of fiber optic cable 402 may be exposed to the sampleliquid or gas. The latter approach may be useful for example, in thecase where the evanescent field (discussed below) extends into thejacket for interaction with the trace species (which has been absorbedor dissolved into the jacket). Removing both the jacket and cladding maynot be the most preferred, however, because of the brittle nature ofinner core 402 c used in certain types of fiber optic cables. A crosssection of a typical fiber optic cable is shown in FIG. 5A.

Bending a total internal reflection (TIR) element changes the angle atwhich the incident electromagnetic wave contacts the reflection surface.In the case of bending an optical fiber about a cylindrical body, theangle of reflection on the surface of the fiber core opposite the bodyis closer to normal, and the penetration depth of the evanescent fieldis increased. By wrapping several turns of optical fiber 402 aroundcylindrical core element 502 (see FIG. 5B), the evanescent fieldpenetration depth is increased and a greater length of fiber can beexposed to the detection fluid in a smaller physical volume. Anexperimental, verification of the improvement in optical fiber sensingthrough varying bending radii is discussed by D. Littlejohn et al. in“Bent Silica Fiber Evanescent Absorption Sensors for Near InfraredSpectroscopy,” Applied Spectroscopy 53: 845-849 (1999).

FIG. 5B illustrates an exemplary sensor 500 used to detect trace speciesin a liquid or gas sample. As shown in FIG. 5B, sensor 500 includescylindrical core element 502 (which may be solid, hollow or otherwisepermeable), such as a mandrel, with a portion of fiber optic cable 402,with cladding 402 b exposed (in this example), wrapped around coreelement 502 over a predetermined length 506. It is also possible tofabricate sensor 500 by wrapping core element 502 where core 402 c offiber optic cable 402 is exposed. The diameter of core element 502 issuch that fiber core 402 c is formed with less than a critical radius r,at which point excess radiation may be lost through fiber core 402 c asit circumscribes core element 502, or fiber integrity is compromised.The critical radius r is dependent on the frequency of the radiationpassing through fiber optic cable 402 and/or the composition of thefiber. In a preferred embodiment of the present invention, the radius ofcore element 502 is between about 1 cm and 10 cm, and most preferably atleast about 1 cm. As illustrated, radiation from fiber 402 is providedat input 504 and extracted at output 508. Cylindrical core element 502may have a spiral groove on its surface in which fiber 402 is placed aswell as a means to secure fiber 402 to cylindrical core element 502.Such securing means may take may forms, such as a screw tapped intocylindrical core element 502, an adhesive, such as epoxy or siliconrubber, etc. The invention may be practiced where sensors 500 areintegral with fiber 402 or may be coupled to fiber 402 utilizingcommercially available fiber-optic connectors.

FIG. 6A illustrates how radiation propagates through a typical fiberoptic cable. As shown in FIG. 6A, radiation 606 exhibits total internalreflection (TIR) at the boundary between inner core 402 c and cladding402 b. There is some negligible loss (not shown) by which radiation isnot reflected, but is absorbed into cladding 402 b. Although FIG. 6A isdescribed as a fiber optic cable, FIG. 6A and the exemplary embodimentsof the present inventions are equally applicable to a hollow fiber, suchas a hollow waveguide, in which cladding 402 b surrounds a hollow core.

FIG. 6B is a cross sectional view of one exemplary embodiment of sensor500 which illustrates the effect of wrapping fiber optic cable 402around core element 502. As shown in FIG. 6B, only jacket 402 a isremoved from fiber optic cable 402. Radiation 606 travels within core402 c and exhibits total internal reflection at the boundary betweeninner core 402 c and the portion of cladding 402 b-1 adjacent coreelement 502 with a negligible loss 609. On the other hand, in thepresence of trace species or analyte 610, evanescent field 608 passesthrough the interface between inner core 402 c and the exposed portionof cladding 402 b-2. This essentially attenuates radiation 606 based onthe amount of trace species 610 present and is called attenuated totalinternal reflection (ATR). It should be noted that if there is no atrace species present having an absorption band compatible with thewavelength of the radiation, radiation 606 is not attenuated (other thanby inherent loss in the fiber).

FIG. 6C is a cross sectional view of another exemplary embodiment ofsensor 500 which illustrates the effect of wrapping fiber optic cable402 around core element 502 with a portion of jacket 402 a remainingintact. As shown in FIG. 6D, only an upper portion of jacket 402 a isremoved from fiber optic cable 402. Similar to the first exemplaryembodiment of sensor 500, radiation 606 travels within core 402 c andexhibits total internal reflection at the boundary between inner core402 c and the portion of cladding 402 b-1 adjacent core element 502 withnegligible loss 609. On the other hand, in the presence of trace speciesor analyte 610 evanescent field 608 passes through the interface betweeninner core 402 c and the exposed portion of cladding 402 b-2.

It is contemplated that the removal of jacket 402 a (in either exampleof sensor 500) may be accomplished by mechanical means, such as aconventional fiber optic stripping tool, or by immersing the portion ofthe fiber cable in a solvent that will attack and dissolve jacket 402 awithout effecting cladding 402 b and inner core 402 c. In the case ofpartial removal of jacket 402 a, the solvent approach may be modified byselectively applying the solvent to the portion of the jacket intendedfor removal.

To enhance the attraction of analyte molecules of the trace species in aliquid sample, a jacket-less portion of the passive fiber optic ring maybe coated with a material to selectively increase a concentration of thetrace species at the coated portion of the fiber optic ring. An exampleof one such coating material is polyethylene. Additionally, antigenspecific binders may be used to coat the fiber to attract a desiredbiological analyte with high specificity.

Referring again to FIG. 4, the radiation that remains after passingthrough sensors 500 continues through fiber loop 402. A portion of thatremaining radiation is coupled out of fiber optic loop 402 by evanescentoutput coupler 416. Evanescent output coupler 416 is coupled toprocessor 420 through detector 418 and signal line 422. Processor 420may be a PC, for example, having a means for converting the analogoutput of detector 418 into a digital signal for processing. Processor420 also controls coherent source 404 through control line 424. Once thesignals are received from detector 418 by processor 420, the processormay determine the amount and type of trace species present based thedecay rate of the radiation received.

Optionally, wavelength selector 430 may be placed between evanescentoutput coupler 416 and detector 418. Wavelength selector 430 acts as afilter to prevent radiation that is not within a predetermined rangefrom being input into detector 418.

Detector 414 is coupled to the output of input coupler 412. The outputof detector 414 is provided to processor 420 via signal line 422 for usein determining when resonant fiber optic ring 402 has receivedsufficient radiation by which to perform trace species analysis.

In the case of detection of trace species or analytes in liquids, theindex of refraction of the liquid must be lower than the index ofrefraction of the fiber optic cable. For example, given a fiber opticcable having an index of refraction of n=1.46, the invention may be usedto detect trace species dissolved in water (n=1.33) and many organicsolvents, including methanol (n=1.326), n-hexane (n=1.372),dichloromethane (n=1.4242), acetone (n=1.3588), diethylether (n=1.3526),and tetrahydrofuran (n=1.404), for example. An extensive list ofchemicals and their respective index of refraction may be found in CRCHandbook of Chemistry and Physics, 52^(nd) edition, Weast, Rober C., ed.The Chemical Rubber Company: Cleveland Ohio, 1971, p. E-201,incorporated herein by reference. There are other types of optical fiberavailable with different indexes of refraction, and the presentinvention can be tailored to a given liquid matrix assuming the opticalfiber has both a higher index of refraction than the liquid andeffectively transmits light in the region of an absorption band by thetarget analyte.

There are many different types of optical fiber currently available. Oneexample is Corning's SMF-28e fused silica fiber which has a standard usein telecommunications applications. Specialty fibers exist that transmitlight at a multitude of different wavelengths, such as a 488 nm/514 nmsingle mode fiber, manufactured by 3M of Austin, Tex. (part no.FS-VS-2614), 630 nm visible wavelength single-mode fiber manufactured by3M of Austin, Tex. (part no. FS-SN-3224), 820 nm standard single-modefiber manufactured by 3M of Austin, Tex. (part no. FS-SN-4224), and0.28-NA fluoride glass fiber with 4-micron transmission, manufactured byKDD Fiberlabs of Japan (part no. GF-F-160). Further, and as mentionedabove, fiber optic cable 402 may be a hollow fiber.

It is contemplated that fiber 402 may be a mid-infrared transmittingfiber to allow for access to spectral regions having much higher analyteabsorption strengths, thereby increasing the sensitivity of theapparatus 400. Fibers that transmit radiation in this region aretypically made from fluoride glasses.

FIG. 7 illustrates a second exemplary embodiment of the presentinvention through which trace species, or analytes, in gases and liquidsmay be detected. In describing FIG. 7, elements performing similarfunctions to those described with respect to the first exemplaryembodiment will use identical reference numerals. In FIG. 7, apparatus700 uses a similar resonant fiber optic ring 408 including fiber opticcable 402 and sensors 500. Radiation from coherent source 404 isprovided to resonant fiber optic ring 408 through optional opticalisolator 406, coupler 410, and evanescent input/output coupler 434.Evanescent input/output coupler 434 may provide a fixed percentage ofradiation from coherent source 404 into resonant fiber optic ring 408,or may be adjustable based on losses present throughout resonant fiberoptic ring 404. In the exemplary embodiment evanescent input/outputcoupler 434 is essentially a reconfiguration of evanescent input coupler412 discussed above with respect to the first exemplary embodiment. It apreferred embodiment, evanescent input/output coupler 434 couples lessthat 1% of the radiation from laser 404 into fiber 402.

Detection of trace species is similar to that described in the firstexemplary embodiment and is therefore not be repeated here.

The radiation that remains after passing through sensors 500 continuesthrough fiber loop 402. A portion of that remaining radiation is coupledout of fiber optic loop 402 by evanescent input/output coupler 434.Evanescent input/output coupler 434 is coupled to processor 420 throughdetector 418 and signal line 422. As in the first exemplary embodiment,processor 420 also controls coherent source 404 through control line424. Once the signals are received from detector 418 by processor 420,the processor may determine the amount and type of trace species presentbased the decay rate of the radiation received.

Optionally, wavelength selector 430 may be placed between evanescentinput/output coupler 434 and detector 418. Wavelength selector 430 actsas a filter to prevent radiation that is not within a predeterminedrange from being input into detector 418. Wavelength selector 430 mayalso be controlled by processor 420 to prevent radiation from coherentsource 404 “blinding” detector 418 during the time period after theradiation from coherent source 404 was coupled into fiber 402.

FIGS. 8A-8D illustrates another exemplary sensor 800 used to detecttrace species in a liquid or gas sample. As shown in FIGS. 8A and 8D,sensor 800 is formed from fiber 801 by tapering the inner core 804 andcladding 805 to create tapered region 802 having tapered inner core 808and tapered cladding 809. The forming of tapered region 802 may beaccomplished using either of two techniques. The first technique isheating of a localized section of fiber 801 and simultaneous adiabaticpulling on either side of the region in which it is desired to formsensor 800. This procedure creates a constant taper in fiber 801. Thistapered fiber can then be for used as a spectroscopic sensor accordingto the first exemplary embodiment, for example. In the second exemplarytechnique, tapered region 802 may be formed by using a chemical agent tocontrollably remove a predetermined thickness of fiber cladding 805 toform tapered cladding 809. A detailed description of a sensor formedusing the second technique is described below with respect to FIGS.10A-10C.

FIG. 8B illustrates a cross section of sensor 800 in the pre taper andpost taper regions. As shown in FIG. 8B, inner core 804 and cladding 805are in an unmodified state. It should be noted, for simplicity, theillustrations and description do not refer to the jacketing of fiberoptic cable 801, though such jacketing is assumed to be in place for atleast a portion of fiber optic cable 801.

FIG. 8C, illustrates a cross section of sensor 800 in tapered region802. As shown in FIG. 8C, tapered inner core 808 and tapered cladding809 each have a significantly reduced diameter as compared to inner core804 and cladding. 805. Tapered region 802 may be of any desired lengthbased on the particular application. In the exemplary embodiment, asshown in FIG. 8D, for example, the length of the tapered region isapproximately 4 mm with a waist diameter 814 of about 12 microns.

Referring again to FIG. 8A, evanescent field 806 in the region of innercore 804 is narrow and confined when compared to enhanced evanescentfield 810 in taped region 802. As illustrated, enhanced evanescent field810 is easily exposed to the trace species (not shown) as discussedabove with respect to the earlier exemplary embodiments and, thus, isbetter able to detect the trace species in region 812.

FIGS. 9A-9C illustrate yet another exemplary sensor 900 used to detecttrace species in a liquid or gas sample. As shown in FIG. 9A, sensor 900is formed from fiber 901 by removing a portion of cladding 905 to createa substantially “D” shaped cross section region 902. The forming of “D”shaped cross section region 902 may be accomplished by polishing oneside of optical fiber cladding 905 using an abrasive, for example. Theabrasive is used to remove cladding 905 in continuously increasingdepths along region 902 to preserve guided mode quality, ultimatelyreaching a maximum depth at the point of minimum cladding thickness 909.This area of lowest cladding thickness represents the region of maximumevanescent exposure 910.

FIGS. 10A-10C illustrate still another exemplary sensor 1000 used todetect trace species in a liquid or gas sample. Sensor 1000 is formedusing the second technique described above with respect to the taperedsensor exemplary embodiment. As shown in FIG. 10A, sensor 1000 is formedfrom fiber 1001 by removing a portion of cladding 1005 using a chemicalagent, known to those of skill in the art, to create tapered region 1002having tapered cladding 1009. It is important that the chemical agentnot be permitted to disturb or remove any portion of the inner core, asthis may introduce significant losses in sensor 1000.

FIG. 10B illustrates a cross section of sensor 1000 in the pre taper andpost taper regions. As shown in FIG. 10B, inner core 1004 and cladding1005 are in an unmodified state. It should again be noted, forsimplicity, the illustrations and description do not refer to thejacketing of fiber optic cable 1001, though such jacketing is assumed tobe in place for at least a portion of fiber optic cable 1001.

FIG. 10C illustrates a cross section of sensor 1000 in tapered region1002. As shown in FIG. 10C, inner core 1004 is not affected whiletapered cladding 1009 has a significantly reduced diameter as comparedto cladding 1005. Tapered region 1002 may be of any desired length basedon the particular application. In the exemplary embodiment, for example,the length of the tapered region is approximately 4 mm with a waistdiameter 1014 of about 12 microns.

Referring again to FIG. 10A, evanescent field 1006 in the region ofinner core 1004 is narrow and confined when compared to enhancedevanescent field 1010 in taped region 1002. As illustrated, enhancedevanescent field 1010 is easily exposed to the trace species (not shown)as discussed above with respect to the earlier exemplary embodimentsand, thus, is better able to detect the trace species in region 1012.

With respect to the above described sensors 800, 900 and 1000, lossescreated in the optical fiber by forming the sensors may be balanced withthe amount of evanescent field exposure by determining the appropriatetaper diameter or polish depth for the desired detection limits prior tofiber alteration. Further, it may be desirable to provide a protectivemounting for sensors 800, 900 and/or 1000 to compensate for increasedfragility due to the respective tapering and polishing operations.

It is contemplated that sensors 800, 900 and/or 1000 may be used ineither as an unrestricted fiber, on a cylindrical core element 502(which may be solid, hollow or otherwise permeable), such as a mandrel(shown in FIG. 5B) or in a loop or bent configuration (not shown).

Sensors 800, 900 and 1000 may be further enhanced by coating the sensingregion with a concentrating substance, such as a biological agent toattract an analyte of interest. Such biological agents are known tothose of ordinary skill in the art. It is also contemplated that severaldetecting regions 800, 900 and/or 1000 may be formed along a length of afiber optic cable to produce a distributed ring down sensor.

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

1. A method for detecting and measuring a trace species in at least oneof a sample gas and a sample liquid, the method comprising: forming acontinuous closed passive optical fiber ring; forming at least onesensor in the closed passive optical fiber ring by tapering a portionthe optical fiber along a length thereof; exposing the tapered portionof the optic fiber to the sample gas or sample liquid; emittingradiation from a coherent source; coupling at least a portion of theradiation emitted from the coherent source into the continuous closedpassive fiber optic ring; receiving a portion of the radiation travelingin the continuous closed passive fiber optic ring; and determining thelevel of trace species in the gas or liquid sample based on a rate ofdecay of the radiation within the continuous closed passive fiber opticring.
 2. A method according to claim 1, further comprising: forming witha predetermined radius at least a portion of the tapered portion of thecontinuous closed passive fiber optic ring based on an absorptionfrequency of the trace species; and exposing the formed portion of theat least one sensor to the sample liquid or sample gas.
 3. A methodaccording to claim 2, further comprising exposing an evanescent field ofthe radiation traveling within the fiber to the sample gas or sampleliquid.
 4. A method according to claim 3, further comprising determiningthe level of the trace species in the sample gas or sample liquid basedon the rate of decay of the radiation in the fiber responsive to anabsorption of the radiation by the trace species.
 5. A method fordetecting and measuring a trace species in at least one of a sample gasand a sample liquid, the method comprising: forming a continuous closedpassive optical fiber ring; forming at least one sensor in thecontinuous closed passive optical fiber ring by removing a portion of acladding of the optical fiber to form a “D” shaped cross section;exposing the “D” shaped cross section portion of the optic fiber to thesample gas or sample liquid; emitting radiation from a coherent source;coupling at least a portion of the radiation emitted from the coherentsource into the continuous closed passive fiber optic ring; receiving aportion of the radiation traveling in the continuous closed passivefiber optic ring; and determining the level of trace species in the gasor liquid sample based on a rate of decay of the radiation within thecontinuous closed passive fiber optic ring.
 6. A method according toclaim 5, further comprising: forming with a predetermined radius atleast a portion of the “D” shaped cross section portion of thecontinuous closed passive fiber optic ring based on an absorptionfrequency of the trace species; and exposing the formed portion of theat least one sensor to the sample liquid or sample gas.
 7. A methodaccording to claim 6, further comprising exposing an evanescent field ofthe radiation traveling within the continuous closed passive fiber opticring to the sample gas or sample liquid.
 8. A method according to claim7, further comprising determining the level of the trace species in thesample gas or sample liquid based on the rate of decay of the radiationin the continuous closed passive fiber optic ring responsive to anabsorption of the radiation by the trace species.